1. Field of the Invention
This invention relates to a coated sand composition for light alloy casting.
2. Description of the Related Art
When casting a complicated structure such as an engine cylinder head in light alloy such as aluminum alloy, cores and the like are shaped from coated sand, that is, casting sand coated with resin binder, and the cores are used in combination with metal molds. The mold using coated sand must have a strength to withstand casting pressure and at the same time must be well disintegrated after casting. However though being apt to disintegrate in the case of casting in cast iron in which the casting temperature is high and resin burns out during casting, such a mold is not satisfactorily disintegrated in the case of casting in light alloy where the casting temperature is low, which makes troublesome sand removal, especially core removal after casting.
Japanese Unexamined Patent Publication No. 2(1990)-175044 discloses a coated sand composition containing therein bromide of wood material such as lignin bromide as disintegrator. The publication says that the bromide promotes disintegration of the sand mold at low temperature and does not give rise to a problem of metal molds. Further it has been well known to use inorganic bromide as disintegrator as disclosed, for instance, in Japanese Patent Publication No. 60(1985)-15417.
However, though the wood bromide may promote disintegration, such a special compound must be synthesized, which is disadvantageous in reduction of cost. Further whether the bromide is organic or inorganic, the bromide cannot avoid being decomposed by heat during casting and generating bromine gas. The bromine gas causes corrosion of the metal molds. The corroded metal mold requires repeated polishing and padding.